The present invention relates to the general field of controlling clearance in a gas turbine between the tips of rotary blades and a stationary ring assembly.
A gas turbine, e.g. a high-pressure turbine of a turbomachine, typically has a plurality of stationary vanes alternating with a plurality of moving blades in a passage for hot gas coming from the combustion chamber of the turbomachine. The moving blades of the turbine are surrounded over the entire circumference of the turbine by a stationary ring assembly. The stationary ring assembly defines the flow steam of hot gas through the blades of the turbine.
In order to increase the efficiency of such a turbine, it is known to reduce to as small as possible the clearance that exists between the tips of the moving blades of the turbine and the facing portions of the stationary ring assembly. In order to do this, means have been devised that serve to vary the diameter of the stationary ring assembly. Such means are generally in the form of annular ducts surrounding the stationary ring assembly and conveying air taken from other portions of the turbomachine. This air is injected against the outside surface of the stationary ring assembly facing away from the stream of hot gas, thereby causing the stationary ring assembly to expand or contract thermally so as to vary its diameter. In general, this thermal expansion or contraction is controlled, depending on the operating speed of the gas turbine, by means of a valve which serves to control the flow rate and the temperature of the air fed to the ducts. The assembly constituted by the ducts and the valve thus forms a box for controlling clearance at the tips of the blades.
Prior art control boxes do not always enable great uniformity of temperature to be obtained over the entire circumference of the stationary ring assembly. A lack of temperature uniformity leads to distortions in the stationary ring assembly which are particularly harmful to the efficiency and the lifetime of the gas turbine.
Furthermore, the air from control boxes that has been injected against the outside surface of the stationary ring assembly needs to be exhausted to the outside. This exhausting of air must be capable of taking place without significantly disturbing the flow of air which is injected against the outside surface of the stationary ring assembly. Nevertheless, in prior art control boxes, it is found that the air that is to be exhausted generally tends to disturb the flow of the air that has been injected, thereby reducing the effectiveness of the box for controlling clearance at the tips of the blades.